1. Field of Invention
The present invention relates generally to a transmission power control system, a control method, a base station and a control station and a storage medium recording a control program. More particularly, the invention relates to a method for determining a balance adjustment start timing upon performing balance adjustment of a transmission power for one mobile station from a plurality of base stations at the occurrence of soft hand-over in cellular communication system.
2. Description of Related Art
In a code division multiplex cellular system, a plurality of channels use the same frequency, a reception power (desired wave power) of a signal at certain channel becomes an interference wave power to be jamming for other channels. Accordingly, in an uplink transmitted from a mobile station to a base station, when the desired wave power is greater than or equal to a predetermined value, interference wave power is increased to reduce capacity of the channel. In order to prevent this, it becomes necessary to strictly control the transmission power of the mobile station. The transmission power control in the uplink is performed in such a manner that the base station measures the desired wave power to compare with a target control value for transmitting a up control instruction for reducing a transmission power (hereinafter referred to as “up transmission power”) of the uplink for the mobile station when the desired power is larger than the target control value, and for transmitting the up control instruction for increasing the up transmission power for the mobile station when the desired wave power is smaller than the target control value. Then, the mobile station increases or decreases the up transmission power according to the up control instruction. Transmission of the up control instruction in the transmission power control is performed using a downlink transmitting from the base station to the mobile station.
On the other hand, even in the downlink, by performing transmission power control so that a ratio between the desired wave power and the interference wave power becomes a predetermined amount to realize high channel capacity. In greater detail, in the transmission power control in the downlink, the mobile station measures a reception quality of the downlink to compare with a target control value for transmitting a down control instruction for reducing transmission power of the downlink (hereinafter referred to as “down transmission power”) for the base station when the reception quality is higher than the target control value, and for transmitting down control instruction for increasing the down control power when the reception quality is lower than the target control value. Then, the base station increases or decreases the down transmission power according to the down control instruction.
However, in this method, when propagation loss from the mobile station to the base station is abruptly increased associating with movement of position of the mobile station, the base station cannot receive the down control instruction from the mobile station. At the same time, even in the mobile station, it can become impossible to receive the up control instruction from the base station. At this time, in the conventional method for only controlling transmission power of the downlink in the mobile station by down control instruction from the base station, when a condition where the propagation loss is increased, is continued, while the base station cannot receive the down control instruction from the mobile station, the base station does not increase the transmission power of the downlink. Therefore, even in the mobile station, it becomes impossible to receive the up control instruction from the base station. Thus, the up transmission power of the signal in the uplink is not increased to continue a condition where communication between the mobile station and the base station is interrupted.
On the other hand, in general, among signals received by the base station, a portion of user information, such as voice, data and so forth are encoded in such a manner that information having relatively long data length are encoded in a lump so that they can be accurately decoded by performing error correction and so forth even when reception error in a moment is caused. Even upon decoding, information having long data length can be decoded in a lump over a relatively long period.
However, during movement of the mobile station at high speed, when high speed transmission power control is performed for maintaining a reception quality constant following to high speed fading fluctuation in the propagation path, even if the user information can be decoded accurately, judgment of the control instruction has to be performed in a moment. Effect of error correction and so forth cannot be obtained in judgment of the control instruction to contain error relatively frequently.
Since error in judgment of such control instruction is caused in relation to increase and decrease of propagation loss, possibility of continuously causing propagation loss becomes relatively high. Then, when judgment error of the control instruction is continuously caused, the base station cannot control the down transmission power of the signal in the downlink according to the down control instruction from the mobile station to make it impossible to accurately receive the signal in the downlink in the mobile station. On the other hand, in this condition, even in the mobile station, since the up control instruction from the base station contained in the signal of downlink cannot receive, the up transmission power of the signal in the uplink is also become impossible to control. At this time, in the base station, among signals in uplink, error of judgment of the down control instruction can be caused frequently, and also, it becomes possible that the user information cannot receive accurately. Even in such case, a condition where communication between the base station and the mobile station is interrupted, is continued.
On the other hand, in the cellular system, when the mobile station moves between cells, there is a soft hand-over technology for switching channels between the cells with simultaneously setting the channels with a plurality of the base stations in the vicinity of the boundary between the cells. This technology is important in the cellular system employing the code division multiplexing system.
Transmission power control in uplink during execution of soft hand-over is important for enabling reception of all up control instructions of all base stations where the propagation loss in uplink potentially becomes minimum.
Therefore, a method for controlling the transmission power of the downlink to equalize the desired wave power from respective base stations in the mobile station, can be considered. However, in this method, since the base station having large propagation loss to the mobile station sets the down transmission power to be large correspondingly to increase interference wave and whereby to reduce capacity of the downlink. As a method for restricting reduction of the capacity of the downlink, there is a method for controlling the down transmission power of respective base stations to be equal to each other.
In this method, a reception power of the up control instruction from the base station having small propagation loss to the mobile station is greater than the reception power of the up control instruction from the base station having large propagation loss to the mobile station. When a difference of the propagation losses is large, probability of reception failure of the up control instruction from the base station having large propagation loss becomes high. In such case, the transmission power in the uplink is mainly controlled by up control instruction from the base station having small propagation loss. Therefore, no significant problem will be arisen. On the other hand, when the difference of the propagation losses is small, it is important to control the up transmission power according to both base stations. In such case, since respective up control instructions can be received at substantially equal power, probability of accurately receiving both up control instructions becomes high. Accordingly, for transmission power control of the uplink, all up control instructions from the base stations which potentially have minimum propagation loss of the uplink can be received.
On the other hand, during execution of soft hand-over, when large and small of the propagation loss from the mobile station to respective base stations is switched at high speed due to fading fluctuation or other cause, the base station having the minimum propagation loss performs transmission at any moment even without switching of the base stations to perform transmission to the mobile station at high speed. At this time, unless the down transmission power of the base stations are equal to each other, reception quality can be increased or decreased upon switching of the base station having the minimum propagation loss to easily cause degradation of the reception quality However, when down transmission power of respective base stations are equal to each other, the reception quality can be maintained substantially constant even when the base stations having the minimum propagation loss is switched to improve reception quality by diversity effect.
In such transmission power control in downlink, the mobile station measures the reception quality of the downlink to compare with the target control value to transmit the down control instruction for reducing the down transmission power for the base station when the reception quality is higher than the target control value, and to transmit the down control instruction for increasing the down transmission power for the base station when the reception quality is lower than the target control value. During execution of soft hand-over, the down control instruction transmitted from the mobile station is received by a plurality of base stations. Then, respective base stations controls the down transmission power to increase or decrease according to the down control instruction. Accordingly, if the initial values of the down transmission power of respective base stations are mutually equal to each other, similar increase or decrease is repeated. If no error is contained in reception of down control instruction, down transmission power can be controlled with maintaining equal condition.
However, in this method, in the base station where the propagation loss to the mobile station becomes minimum, down control instruction from the mobile station can be received in substantially accurate. However, in the base station having relatively large propagation loss in transmission from the mobile station, reception of down control instruction from the mobile station can be frequently failed for small transmission power of the down control instruction. Accordingly, it becomes impossible maintain the down transmission powers of respective base stations equal to each other.
Therefore, during execution of the soft hand-over, a transmission power control method in the cellular communication system, in which substantially equal power can be transmitted from respective each base station even if error is caused in reception of the down control instruction in respective base stations, and whereby high channel capacity can be attained, has been proposed in Japanese Unexamined Patent Publication No. Heisei 11-340910.
FIG. 6 schematically shows the construction of the cellular communication system. In FIG. 6, service area is divided into first and second cells 11 and 12. In the first and second cells 11 and 12, a first base station (#1) 21 and a second base station (#2) 22 are respectively arranged. Also, first and second mobile stations 61 and 62 are present in the first and second cells 11 and 12. The first and second base stations 21 and 22 are connected to a common control station 71. The common control station 71 is connected to a communication network (not shown) constituted of other control station. It should be noted that while not illustrated, the cellular communication system includes other large number of base stations, and in each cell, large number of mobile stations are present.
The first and second base stations 21 and 22 transmit first and second pilot signals 31 and 32 at respectively given transmission power. Each mobile station 61, 62 has SIR (a ratio of desired wave and an interference power) measuring equipment for measuring a power of the pilot signal for respectively measuring reception power of the first and second pilot signals 31 and 32. The mobile station switches the measuring equipment of the pilot signal in a short period per slot (time slot) as shown in FIG. 7 and measures the pilot signals of a plurality of base station once per frame. In the example of FIG. 7, six slots are present within one frame and permit measurement of the pilot signals from the six base stations at the maximum. In FIG. 6, the reference numerals 41, 41a, 41b, 42 denote signals in downlink and 51, 52 denote signals in uplink.
Next, discussion will be given for the transmission power control for downlink in the cellular communication system shown in FIG. 6 with reference to FIG. 8. FIG. 8 is a flowchart showing operation of the base station for determining the down transmission power in the downlink in response to the down control instruction from the mobile station during soft hand-over. Here, the down transmission power P is expressed by a decibel value.
When the base station initiates soft hand-over with the mobile station, if the base station in question is the primary base station which has been transmitting to the mobile station before, the down transmission power P maintains the preceding value of the transmission power to the mobile station. On the other hand, if the base station is an auxiliary base station newly starting transmission to the mobile station, the down transmission power P is set at an initial value P0. The primary station and the auxiliary station are notified a frame number to initiate soft hand-over from the control station 71. The initial value P0 may be an arbitrary value falling within a control range of the down transmission power.
At first, when a transmission power balance control message between a plurality of base stations arrives from the control station 71, the base station resets a frame counter I=0 (step S201). The frame counter I is incremented by 1 per frame (step S202). Here, the down control instruction (TPC: Transmission Power Control) is notified from the mobile station at a given interval. When the newly notified down control instruction is present (step S203) and the down control instruction designates increasing of the power (step S204), the down transmission power P is increased for a predetermined value ΔP (step S205), and when the down control instruction designates decreasing of the power, the transmission power P in the downlink is decreased for the predetermined value ΔP (step S206).
The foregoing processes S203 to S206 are repeated for frame number Nperiod as a predetermined balance adjusting period. After expiration of the predetermined balance adjusting period (step S207), namely when I=Nperiod is established, a difference (C−P) between a predetermined reference power (referred to as target value or reference value) C and the down transmission power P before updating, is multiplied with a coefficient (1−r) to integrate the down transmission power P (step S208).P=P+(1−r)(C−P)
The coefficient r is a predetermined value within a range greater than or equal to 0 and less than 1. On the other hand, C is an intermediate power between the maximum power Pmax and the minimum power Pmin of the transmission power P.
If the updated transmission power is greater than the maximum power Pmax, the down transmission power P is set at the maximum power Pmax (steps S209, S210). When the updated transmission power P is smaller than the minimum power Pmin, the down transmission power P is set at the minimum power Pmin (steps S211, S212). Then, the process is repeated from the step S202.
In this method, upon timing of initiation of the soft hand-over, since the initial values of the down transmission power of the primary base station and the auxiliary base station are different, there is a difference |P1−P2| between the down transmission power P1 of the primary base station and the down transmission power P2 of the auxiliary base station. On the other hand, upon failure of reception of the down control instruction in one or more base stations, the difference |P1−P2| of these transmission powers P1 and P2 can be increased. However, in a portion of control through steps S203 to S206, namely a portion for increasing and decreasing the down transmission power by the down control instruction from the mobile station, respective base stations receive the same down control instruction. Therefore, respective base stations do not fail reception of the down control instruction, the down transmission power P1 and P2 are increased or decreased in similar manner so as not to vary the difference |P1−P2| of these down transmission powers P1 and P2.
On the other hand, at the same time, per frame number of I=Nperiod, the primary base station and the auxiliary base station simultaneously update the down transmission powers P1 and P2 as P1+(1−r)(C−P1), P2+(1−r)(C−P2). Therefore, the difference |P1−P2| of the down transmission powers P1 and P2 becomes r|P1−P2|. Thus, the difference |P1−P2| of the down transmission power becomes r times per the period Nperiod. Since the coefficient r is smaller than 1, the difference of the control amount is decreased in geometric manner to be converged to 0 unless the difference |P1−P2| of the down transmission powers is increased due to reception error of the new down control instruction. On the other hand, even if the difference |P1−P2| of the down transmission power is increased due to occurrence of reception error of the new down control instruction, the difference |P1−P2| can be decreased. Accordingly, even by failure of reception of the down control instruction, the transmission powers Pi (i=1, 2) in the downlink can be adjusted to substantially equal value between the base stations without direct exchange of information concerning the down transmission power between the base stations.
Namely, after increasing or decreasing the down transmission power by the control at steps S203 to S206, the difference of the down transmission power of a plurality of base stations can be decreased (balance adjustment), and thus the down transmission power is updated to be closer to the reference power C determined in common for a plurality of base stations.
As set forth above, during execution of the soft hand-over by the mobile station up control instruction of the transmission power control of the uplink is transmitted at substantially equal power between the base stations from each base station to the mobile station. Therefore, when the propagation losses from respective base stations to the mobile stations are substantially the same, when the propagation loss in the uplink can be minimum in any base station, all up control instructions can be received in the mobile station. Accordingly, the mobile station can control the up transmission power so that the desired wave power will not become excessive for any base station.
On the other hand, during execution of the soft hand-over, even if large and small relationship of the propagation losses from the mobile station and respective base stations is switched at high speed due to fading fluctuation or the like, owing to diversity effect to maintain the reception quality substantially constant, the reception quality in the mobile station can be further improved. By controlling the up transmission power so that the desired wave power will not become excessive, channel capacity of the uplink can be increased. On the other hand, if the reception quality in the mobile station can be improved by diversity effect, the channel capacity of the downlink at constant reception quality, can be increased.
As set forth above, in each base station, the transmission power is decreased for an adjusting amount in the balance adjusting period for the transmission power. The adjusting amount is derived as a predetermined ratio of the difference between the transmission power at starting timing of the adjusting period and the reference value C. This manner is illustrated in FIG. 9A. In the drawings of FIGS. 9A and 9B, Pbali (i=1, 2) is the power amount to be adjusted, and T1, T2 and T3 are adjusting timings. It should be noted that the drawing shows the width of Pbali with taking r=0.
Since the transmission power in each base station is increased or decreased according to the same transmission power control instruction (TPC bit) from the mobile station, if reception error is not contained in the transmission power control instruction, the transmission power of the base station can be increased or decreased in similar manner. At this time, if the start timing of the adjusting period is the same timing at respective base stations, when the transmission power of one of two base stations is large (P1>P2), the difference Pbal between the transmission power at the start timing of the adjusting period and the reference value C is also large in comparison with that of other base station (Pbal1>Pbal2), the transmission power (P1) of one of two base stations is decreased significantly during the adjusting period. As set forth, the base station having large transmission power is significantly decreased the transmission power to reduce the difference of the transmission power between the base stations to effect balance adjustment.
However, as shown in FIG. 9B, when the start timing of the adjusting period is difference between the base stations such as T1 and T1′, since the transmission power according to the transmission power control instruction is constantly varied, if the transmission power of one of two base stations becomes greater than the other base station (P1>P2), the adjustment start timing T1 of the former base station is a moment where the transmission power is relatively small, and the adjustment start timing of later base station is a moment where the transmission power is relatively large, the difference between the transmission power at start timing of the adjusting period and the reference C becomes large in the later base station than the former base station (Pbal1<Pbal2) to significantly reduce the transmission power of the adjusting period. Therefore, the difference of the transmission powers of the base stations becomes large to cause difficulty in establishing balance of the power. As a result, equalization of the transmission powers between the base stations cannot be achieved to decrease channel capacity.
As set forth above, the phenomenon where the start timings of the adjusting periods are different such as T1 and T1′, is caused by the fluctuation of arrival timings of the control messages for transmission power balance adjustment to respective base stations 21 and 22 from the control station 71 due to fluctuation of transmission delay between the control station and the base station. The reception timings of the control messages for conventional power balance adjustment shown in FIG. 2A is shown in the case where the reception timings are different between the base stations. In FIG. 2A, Nperiod=2 is taken as balance adjusting period and eight frame number of 0 to 7 are taken to repeat. As set forth above, in the prior art, a difference of the reception timings of the power balance control messages is constantly continued subsequently. Therefore, calculation timings of Pbal between the base stations are shifted constantly. As shown in FIG. 9B, reversal of Pbal1 and Pbal2 can be caused.